THE SKELETON AND TEETH 14It
main ones. Both premolars are broad and oval in outline. The first lower premolar tooth is large and pyramidal; it has one large cusp, and its outer surface is worn and polished by friction against the upper canine tooth. The second lower premolar has two almost equal cusps lying in the labial and lingual positions.
The upper molar teeth have large, almost prismatic cusps, which are less crenated than those of the Chimpanzee and Orang ; and the enamel is characterized by its crystalline appearance. The outline of the crowns is like the figure 8, with variations in the relative sizes of the anterior and posterior parts. The crowns have the usual four cusps, and the metacones and protocones are connected, except perhaps on the third tooth, which is degenerating. The teeth decrease in the order '2,1, 3. There are three roots in each case. Keith (100) points out that any upper molar tooth over 12 mm. in length belongs to a Gorilla, and any upper molar under 1*2 mm. in length belongs to a Chimpanzee. The lower molar teeth, are more elongated than in the Chimpanzee and Orang, and they increase in size in the order 1, 2, 3* Most have five cusps, but an additional sixth cusp may be present on the posterior part. The protoconid and metaconid may be connected by a crest.
The third molar and canine teeth usuallv cut
Dental anomalies, particularly supernumerary teeth, are more frequent than in the Chimpanzee.
Much assistance has recently been obtained in anthropological work from the examination of the teeth by X-rays, for that method has enabled us to settle whether